Although so-called radiography employing silver halide photography is conventionally used to obtain radiographic images, there also has been developed a radiographic imaging method not using silver halide photographic materials. It is the method with which radiographic images can be recorded by employing a radiation image conversion panel having a stimulable phosphor layer on a support.
In order to record a radiographic image by using a radiation image conversion panel, radiation that has been transmitted through an object is absorbed by a stimulable phosphor layer and a radiation energy depending on a radiation transmittance density on each part of the object is accumulated. A radiation energy accumulated in a stimulable phosphor can be emitted as a stimulated luminescence by irradiating the stimulable phosphor layer with electromagnetic waves (stimulating light) such as visible light and infrared radiation to excite on a time series basis. This signal based on light intensity can be reproduced as a visible image on a recording material such as a silver halide photographic sensitized material and a display apparatus such as CRT by converting this signal into an electric signal by, for example, photoelectric conversion.
It is described in JP-A No. 2003-028995 (hereinafter, the term, JP-A refers to Japanese Patent Application Publication) that a high-sensitivity radiation image conversion panel can be manufactured by using a stimulable phosphor expressed by the following formula (1), especially the stimulable phosphor exhibiting a numerical value in the range of 0.003≦e≦0.005.M1X·aM2X′2·bM3X″3:eA  (1)where M1 represents at least one alkali metal selected from the group consisting of Li, Na, K, Rb and Cs; M2 represents at least one divalent metal selected from the group consisting of Be, Mg, Ca, Sr, Ba, Zn, Cd, Cu and Ni; M3 represents at least one trivalent metal selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Al, Ga and In; each of X, X′ and X″ represents at least one halogen selected from the group consisting of F, Cl, Br and I; A represents at least one metal selected from the group consisting of Eu, Tb, In, Ga, Cs, Ce, Tm, Dy, Pr, Ho, Nd, Yb, Er, Gd, Lu, Sm, Y, Tl, Na, Ag, Cu and Mg; and a, b and e respectively show numerical values within ranges of 0≦a<0.5, 0≦b<0.5 and 0.0001<e≦1.0.
Especially, divalent Eu activator-alkali metal fluoride type phosphor containing I, divalent Eu activator-alkali metal halide type phosphor containing I, rare earth element activator-rare earth oxyhalide type phosphor containing I, and Bi activator-alkali metal halide type phosphor containing I among stimulable phosphors mentioned above lead to a stimulated luminescence in enhanced luminance.
A radiation image conversion panel in which those stimulable phosphors are used has an advantage of accumulation of radiographic images once again after scanning and a repetitive use because an accumulated energy is released by scanning of a stimulating light after the radiographic image information is accumulated. Thus, this radiation image conversion method is of advantage from the aspect of resource conservation and economic efficiency because the radiation image conversion panel is repeatedly used, while radiographic sensitized materials are consumed for each radiographing in the case of a conventional radiographic imaging method.
Those radiation image conversion panels are used in quantities for a medical X-ray image diagnosis apparatus and so forth. In many cases, a stimulable phosphor is laminated on a support in a radiation image conversion panel and the radiation image conversion panel is often stored in a cassette for radiography to be handled.
A cassette for radiography (which is thereinafter called “cassette”) is a flat enclosure which capable of storing a radiation image conversion panel which prevents not only a physical damage of a stimulable phosphor at the time of transportation or radiographing, but also an annihilation of accumulated image information caused by emitting a stimulating light to a stimulable phosphor after radiographing. As shown in FIG. 3, radiation image conversion panel 30 is stored in cassette 40 with stimulable phosphor 31 arranged to face front plane 41 of cassette 40.
Radiographing is carried out by arranging object 50 to face the outside surface of front plane 41 of cassette 40 in which radiation image conversion panel 30 is stored and by irradiating radiation transmitted through object 50 to radiation image conversion panel 30 in cassette 40. Compared with a method using silver halide, this radiography method makes it possible to obtain radiation images having sufficient amount of information, by an extremely small amount of radiation exposure.
A radiation image conversion panel in this method is used by repeating those steps of irradiation of radiation (recording a radiation image), irradiation of stimulating light (reading a radiation image recorded), and irradiation of erasure light (erasing a remaining radiation image). A conveying means such as a belt, a roller, and so forth is employed to transfer to each step.
Though a radiation image recording-reproducing method has many excellent advantages as described above, it is desired that the radiation image conversion panel used in the foregoing method is as highly sensitive as possible and it gives images having excellent image quality (sharpness, graininess, and so forth).
It has usually been thought that sharpness of an image obtained by a radiation image conversion panel becomes more enhanced with a thinner protective layer. But, a repetitive use tends to cause a degradation of durability with appearance of scratches and cracks generated on the surface of a protective layer when a protective layer is thin.
Especially in the case of a cassette type radiation image conversion panel, foreign matters sometimes adhere to a protective layer situated on the phosphor surface, when it is repeatedly transported by a conveying means. The surface of the protective layer is often scratched by scraping on the surface when those foreign matters are hard. A protective layer having a high scratch resistance property is required since it is known that occurrence of a scratched protective layer adversely affects a picture image quality.
Thus, applicable materials used for a protective layer are selected in such a way to give to the protective layer functions necessary as a protective layer such as scratch resistance and transport durability, while controlling a decline of sharpness by making the protective layer to be as thin as possible. For example, those having strength like that of polyethylene terephthalate and high light transmittance are used, several kinds of resins are combined, a layer of a resin compound containing a fluorine type resin which possesses a high scratch resistance is coated, or a protective layer itself is made to be of a multilayer structure. (Refer to JP-A No. 10-82899)
There is also present a radiation image conversion panel having enhanced sharpness, high transport durability, and at the same time a high sensitivity by employing an absorbing layer of a stimulating light in a protective layer and specifying further a surface roughness. (Refer to JP-A No. 2002-122698)